In optics, a diffraction grating is an array of fine, parallel, equally spaced grooves (“rulings”) on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called “orders,” or “spectral orders.”
The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter.
Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle θ, the deviation of the diffracted ray from the direction predicted by geometric optics, is given by the following equation, where m is the spectral order, λ is the wavelength, and d is the spacing between corresponding parts of adjacent grooves:
  θ  =      ±                  sin                  -          1                    ⁡              (                              m            ⁢                                                  ⁢            λ                    d                )            
Because the angle of deviation of the diffracted beam is wavelength-dependent, a diffraction grating is dispersive , i.e. it separates the incident beam spatially into its constituent wavelength components, producing a spectrum.
The spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order. Diffraction gratings are often used in monochromators and other optical instruments.
By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called “blazing.”
Originally high resolution diffraction gratings were ruled. The construction of high quality ruling engines was a large undertaking. A later photolithographic technique allows gratings to be created from a holographic interference pattern. Holographic gratings have sinusoidal grooves and so are not as bright, but are preferred in monochromators because they lead to a much lower stray light level than blazed gratings. A copying technique allows high quality replicas to be made from master gratings, this helps to lower costs of gratings.
A planar waveguide reflective diffraction grating includes an array of facets arranged in a regular sequence. The performance of a simple diffraction grating is illustrated with reference to FIG. 1. An optical beam 1, with a plurality of wavelength channels λ1, λ2, λ3 . . . , enters a diffraction grating 2, with grading pitch Λ and diffraction order m, at a particular angle of incidence θin. The optical beam is then angularly dispersed at an angle θout depending upon wavelength and the order, in accordance with the grating equation:mλ=Λ(sin θin+sin θout)  (1)
From the grating equation (1), the condition for the formation of a diffracted order depends on the wavelength λN of the incident light. When considering the formation of a spectrum, it is necessary to know how the angle of diffraction θNout varies with the incident wavelength θin. Accordingly, by differentiating the equation (1) with respect to θNout, assuming that the angle of incidence θin is fixed, the following equation is derived:∂θNout/∂λ=m/Λ cos θNout  (2)
The quantity dθNout/dλ is the change of the diffraction angle θNout corresponding to a small change of wavelength λ, which is known as the angular dispersion of the diffraction grating. The angular dispersion increases as the order m increases, as the grading pitch Λ decreases, and as the diffraction angle θNout increases. The linear dispersion of a diffraction grating is the product of this term and the effective focal length of the system.
Since light of different wavelengths λN are diffracted at different angles θNout, each order m is drawn out into a spectrum. The number of orders that can be produced by a given diffraction grating is limited by the grating pitch Λ, because θNout cannot exceed 90°. The highest order is given by Λ/□□. Consequently, a coarse grating (with large Λ) produces many orders while a fine grating may produce only one or two.
The free spectral range (FSR) of a diffraction grating is defined as the largest bandwidth in a given order which does not overlap the same bandwidth in an adjacent order. The order m is important in determining the free spectral range over which continuous dispersion is obtained. For a given input-grating-output configuration, with the grating operation at a preferred diffraction order m for a preferred wavelength λ, other wavelengths will follow the same path at other diffraction orders. The first overlap of orders occurs whenmλm=(m+1)λm+1  (3)
                              λ                      m            +            1                          =                              m            ⁢                                                  ⁢                          λ              m                                            (                          m              +              1                        )                                              (        4        )                                Δλ        =                              λ            m                                m            +            1                                              (        5        )            
A blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a blaze angle w, as shown in FIG. 1. The selection of the blaze angle w offers an opportunity to optimize the overall efficiency profile of the diffraction grating, particularly for a given wavelength.
Planar waveguide diffraction based devices provide excellent performance in the near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM). In particular, advancements in Echelle gratings, which usually operate at high diffraction orders (40 to 80), high angles of incidence (approx 60°) and large grading pitches, have lead to large phase differences between interfering paths. Because the size of grating facets scales with the diffraction order, it has long been considered that such large phase differences are a necessity for the reliable manufacturing of diffraction-based planar waveguide devices. Thus, existing devices are limited to operation over small wavelength ranges due to the high diffraction orders required (see equation 5).
Furthermore, for diffraction grating-based devices fabricated in a planar waveguide platform, a common problem encountered in the prior art is polarization dependent loss arising from field exclusion of one polarization caused by the presence of conducting metal S (a reflective coating) adjacent to the reflective facets F.
An optical signal propagating through an optical fiber has an indeterminate polarization state requiring that the (de)multiplexer be substantially polarization insensitive so as to minimize polarization dependent losses. In a reflection grating used near Littrow condition, and blazed near Littrow condition, light of both polarizations reflects equally well from the reflecting facets (F in FIG. 1). However, the metalized sidewall facet S introduces a boundary condition preventing light with polarization parallel to the surface (TM) from existing near the surface. Moreover, light of one polarization will be preferentially absorbed by the metal on the sidewall S, as compared to light of the other polarization. Ultimately, the presence of sidewall metal manifests itself in the device performance as polarization-dependent loss (PDL).
There are numerous methods and apparatus for reducing the polarization sensitivity of diffraction gratings. Chowdhury, in U.S. Pat. Nos. 5,966,483 and 6,097,863 describes a reduction of polarization sensitivity by choosing to reduce the difference between first and second diffraction efficiencies of a wavelength within the transmission bandwidth. This solution can be of limited utility because it requires limitations on election of blaze angles and blaze wavelength.
Sappey et al, in U.S. Pat. No. 6,400,509, teaches that polarization sensitivity can be reduced by including reflective step surfaces and transverse riser surfaces, separated by a flat. This solution is also of limited utility because it requires reflective coating on some of the surfaces but not the others, leading to additional manufacturing steps requiring selective treatment of the reflecting interfaces.
The free spectral range of gratings is proportional to the size of the grating facets. It has long been thought that gratings with a small diffraction order could not be formed reliably by means of photolithographic etching, because low order often implies steps smaller or comparable to the photolithographic resolution. The photolithographic resolution and subsequent processing steps blur and substantially degrade the grating performance. Therefore, the field of etched gratings has for practical reasons limited itself to reasonably large diffraction orders typically in excess of order 10. Devices with orders ranging close to order 1 have long been thought to be impractical to realize.
An object of the present invention is to overcome the shortcomings of the prior art by providing a planar waveguide reflective diffraction grating providing an increased bandwidth, due to operating at a relatively low order, with very little PDL, due to very small sidewall lengths.